1. Field of the Invention
The present invention relates to solid-state image pickup apparatus and an image pickup system.
2. Description of the Related Art
In recent years, a great advance has been made in technology associated with a CMOS-type solid-state image pickup apparatus. In the CMOS-type solid-state image pickup apparatus, each pixel thereof includes a photodiode, a floating diffusion (hereinafter referred to as FD) region that converts an electric charge transferred via a transfer transistor into a voltage signal, an amplifier, and a reset transistor that resets the FD region. The CMOS-type solid-state image pickup apparatus tends to increase in the number of pixels and decrease in pixel size every year. To meet this tendency, Japanese Patent Laid-Open No. 9-46596 discloses a technique in which transfer transistors are provided for respective photodiodes and electric charges generated by the photodiodes are transferred to one FD region. This technique allows a reduction in the number of amplification transistors and the number of reset transistors per pixel, and thus allows an increase in the area size for the photodiode.
A further description is given below as to the technique disclosed in Japanese Patent Laid-Open No. 9-46596 in which electric charges generated in a plurality of photodiodes are transferred to a shared FD region. In a case where one FD region is shared by two photodiodes, the FD region is first reset when the reset transistor is turned on. Note that the resetting is continued as long as the reset transistor is in the on-state. At a time at which a signal is to be read out from a first photodiode, the reset transistor is turned off, and a transfer transistor connected to the first photodiode is turned on to transfer the signal charge to the FD region. Thereafter, the reset transistor is again turned on to reset the FD region. After the resetting, the reset transistor turns off. The transfer transistor connected to the second photodiode is then turned on to transfer the signal charge to the FD region. The process described above is performed repeatedly to read signals from all pixels.
In the technique disclosed in Japanese Patent Laid-Open No. 9-46596, at a time immediately before the electric charge of the first photodiode is transferred, the potential of the FD region is equal to the FD potential obtained after the reset transistor has remained in the on-state for a long period. On the other hand, the potential of the FD region given at a time immediately before the electric charge of the second photodiode is transferred is equal to the FD potential obtained after the reset transistor has remained in the on-state for a short period after the charge transfer of the first photodiode is complete.
Thus, a difference between the two FD potentials occurs for reasons described below. In the solid-state image pickup apparatus, with increasing signal reading rate, the reading time per pixel decreases. More specifically, for example, the reset time immediately before the signal charge is transferred from the second photodiode is as short as a few μsec or shorter. As a result, the reset time before the signal is read from the second photodiode is shorter than the reset time before the signal is read from the first photodiode. Thus, the reset operation is ended before complete resetting is achieved. Therefore, a difference occurs between two photodiodes in terms of reset potential in a state immediately before the reading is performed. This difference results in a difference in image output, which results in a problem that a reduction in image quality occurs.